1. Field of the Invention
The present invention relates to a technique of assisting in safe driving by determining a state of attention (e.g., a state of being focused on driving or a state of being distracted) of an operator performing operation of a vehicle, such as driving an automobile, on the basis of an electroencephalogram.
2. Description of the Related Art
In recent years, in the development of safe driving assistance techniques, there are increasing needs for techniques of grasping the physical state or psychological state of a driver in real time and providing assistance which is adapted to the state of the driver. As a technique of objectively and quantitatively evaluating the state of a driver, attempts of quantifying the arousal level by using physiological indices, e.g., an electroencephalogram or nictitation, are being made. For example, Japanese Laid-Open Patent Publication No. 7-108848 discloses a technique of estimating the arousal level of a driver from a hypnagogic waveform pattern or an α wave component of the electroencephalogram.
However, the inventors consider it insufficient to employ the mere arousal level to define the state of a driver during driving. The reason is that the arousal level alone does not help define a state where attention is not directed to driving (a so-called state of distraction) although one is in wakeful state. Therefore, what is needed is not only the conventional dozing-off detection based on the arousal level, but also a technique of measuring and evaluating a specific state of attention with respect to driving (e.g., a “looking-aside of the conscious”, which is the conscious being not focused on driving because of thinking about something else, or being absorbed in music or a conversation, for example).
In recent years, studies are undertaken to examine how much attention is being paid to a visual object by using an eye fixation related potential (EFRP) of the electroencephalogram. This method makes it possible to examine a state of attention to driving, including a looking-aside of the conscious. As used herein, an “eye fixation related potential” refers to a transient potential fluctuation in the brain which occurs in relationship with the end time of a rapid oculomotor movement (saccade), i.e., the start time of an eye fixation, while a person is working on a task or looking at things at liberty. Among the components of an eye fixation related potential, a positive component which appears more dominantly in the occiput than in the sinciput is especially referred to as a “lambda response”. A lambda response is known to fluctuate with the degree of attention and concentration onto a visual object.
For example, in Japanese Laid-Open Patent Publication No. 2007-125184, in an environment where large and small saccades variously occur, the saccades are classified based on saccade size or patterns of line-of-sight movement, and an eye fixation related potential is calculated. Then, a degree of attention and concentration is evaluated from a specific component (e.g., the amplitude value of an eye fixation related potential component corresponding to a lambda response) of the calculated eye fixation related potential.
However, since the manner in which an electroencephalogram waveform appears is greatly affected by the individual difference of each driver, the conventional technique described in Japanese Laid-Open Patent Publication No. 2007-125184 cannot accurately determine a state of attention to driving.
This will be specifically described with reference to FIGS. 1A to 1C. FIGS. 1A to 1C show results of an experiment conducted by the inventors concerning the eye fixation related potential. FIG. 1A shows a waveform which is obtained as an arithmetic mean of the waveforms of all of test subjects measured at the occiput; FIG. 1B is a waveform which is obtained as an arithmetic mean of waveforms of test subject A taken through a plurality of measurements; and FIG. 1C is a waveform which is obtained as an arithmetic mean of waveforms of test subject B taken through a plurality of measurements. In each of the graphs of FIGS. 1A to 1C, the horizontal axis represents time (latency) based on the start time of an eye fixation defined as 0 milliseconds, in units of milliseconds. Moreover, the vertical axis represents potential (EERP amplitude) in units of μV, where downward reads positive. The solid line indicates the eye fixation related potential in a state of being focused on driving, whereas the dotted line indicates the eye fixation related potential in a state where attention is not paid to driving (i.e., a state of distraction).
According to the graph of FIG. 1A, the amplitude of a positive component (lambda response) which appears near about 100 milliseconds based on the eye fixation start time as a starting point is as large as 3.4 μV when focused on driving, and as small as 1.2 μV when distracted. It can be seen that the amplitude value of lambda response increases or decreases in accordance with the state of attention, as indicated by conventional knowledge. When determining the state of attention to driving of each test subject by using the amplitude value of a lambda response, it is desirable to set a certain threshold and determine a state of focused driving if the amplitude value of the lambda response is equal to or greater than the threshold, or determine a state of distraction if it is smaller than the threshold. In test subject A, the threshold may be set near 0.9 μV according to the graph of FIG. 1B. When an optimum threshold for determining a state of attention is determined based on a graph which is liable to individual differences, the value will greatly differ among test subjects. Thus, it will be seen that the state of attention to driving cannot be accurately determined without taking such individual differences into consideration.
One method of adjusting for such individual differences is previously performing an adjustment as to the determination criterion of each test subject, i.e., so-called calibration. For example, Japanese Laid-Open Patent Publication No. 2005-34620 proposes a method of adjusting for individual differences when using an electroencephalogram interface for distinguishing an option that a test subject wishes to select from among a plurality of options by utilizing an event-related potential of the electroencephalogram. In this method, one test subject is asked to previously perform about 100 runs of a task of selecting one option from among a plurality of options by using the electroencephalogram interface. Then, from the electroencephalogram data obtained from each task, the system learns characteristic features of the electroencephalograms of the respective individuals.
A similar method might also be possible for determining a state of attention to driving by using an electroencephalogram. However, the requirement of a complicated calibration task prior to driving is troublesome and is a lot of burden on the driver. Furthermore, it is practically impossible for actually driving to be conducted in a state of distraction on a public road for the sake of a previous calibration task, which makes it difficult to acquire a sufficient amount of data for learning in advance.